In transmission of a speech signal between a mobile station (MS) and a network, it is important to optimise the coding of the signal both to provide robust transmission in order to deal with variability of channel quality over the air interface and to provide high quality speech. Transmission can be made more robust in poor channel conditions by increasing the amount of error correction provided in channel coding. Speech quality can also be increased by using a higher bit rate in speech transmission. Since the gross channel bit rate is generally fixed, a balance needs to be struck between the proportions of it which are assigned to channel coding and to speech coding.
The European Telecommunications Standards Institute (ETSI) has standardised a speech service which is intended to deal with this problem. This is referred to as the Adaptive Multi Rate (AMR) speech service and, in one form, is defined in ETSI standards. In the AMR speech service the bit rates assigned to speech and channel coding are adaptively varied based on the channel quality. This is done independently in both the up-link (mobile to network) and in the down-link (network to mobile) directions. The AMR speech service has also been chosen as the mandatory speech service for third generation (3G) telecommunication systems.
FIGS. 1A and 1B show wireless telecommunications system capable of providing an AMR speech service according to the standards referred to above. A and B show how FIGS. 1A and 1B are connected to each other. The system comprises a network and a mobile station (MS). The network comprises a transcoder (TC) and a base transceiver station (BTS). A signal is transmitted from the MS to the network over an up-link radio channel and from the network to the MS over a down-link radio channel.
In the up-link direction, in the MS speech is speech encoded in a multi-rate speech encoder, channel encoded in a multi-rate channel encoder and the resulting encoded signal is then transmitted across the air interface. In the network, the encoded signal is decoded in a multi-rate channel decoder in the BTS and then speech decoded in a multi-rate speech decoder in the TC.
In the down-link direction, speech is encoded in a multi-rate speech encoder in the TC, channel encoded in a multi-rate channel encoder in the BTS and the resulting encoded signal is then transmitted across the air interface. In the MS, the encoded signal is channel decoded in a multi-rate channel decoder and then speech decoded in a multi-rate speech decoder.
Both for up-link and down-link, information about the quality of the radio link is derived by estimating the current channel state. Based on the channel state, and also taking into consideration possible constraints from network control, such as network load, encoding and decoding are adaptively varied by codec mode control. The codec mode control, in the form of a link adaptation block containing up-link and down-link mode adaptors, is located on the network side. The codec mode control selects the codec modes to be applied.
In this sense the term “codec” is referring to corresponding encoder/decoder pairs on opposite sides of the air interface. Therefore, in up-link, the codec is the multi-rate speech encoder and multi-rate channel encoder in the MS and the multi-rate channel decoder and multi-rate speech decoder in the network. In down-link, the codec is the multi-rate speech encoder and the multi-rate channel encoder in the network and the multi-rate channel decoder and multi-rate speech decoder in the in the MS.
The channel mode which is used (TCH/AFS (adaptive full rate traffic channel for speech) or TCH/AHS (adaptive half rate traffic channel for speech) is controlled by the network. Up-link and down-link always have the same channel mode.
A quality indicator is derived at the side which receives the transmitted encoded signal. The quality indicator is derived by a quality measurement unit.
For up-link adaptation, the quality indicator from an up-link quality measurement unit is directly fed into an up-link mode control unit located in the network. This mode control unit quantises the quality information according to certain threshold values and generates, taking into consideration possible constraints from network control, a Mode Command MCu indicating the codec mode to be used in the up-link path (in the up-link direction). The Mode Command is then transmitted in-band across the air interface to the MS and it is provided to the multi-rate speech encoder in the MS. The bit rate of speech encoding is changed in response to this command. The multi-rate speech encoder provides a Mode Indicator MIu to the multi-rate channel encoder in the MS. The bit rate of channel encoding is changed in response to this indication. The bit rate changes to the multi-rate speech encoder and to the multi-rate channel encoder are linked so that, in total, they equal the up-link channel gross bit rate. The Mode Indicator is then transmitted in-band across the air interface to the network and it is provided to the multi-rate channel decoder in the network. The bit rate of the channel decoder is changed in response to this Mode Indicator. The multi-rate channel decoder provides the Mode Indicator to the multi-rate speech decoder in the network (particularly in the transcoder of the network). The bit rate of the speech decoder is changed in response to the Mode Indicator. The bit rate changes to the multi-rate channel decoder and to the multi-rate speech decoder are adjusted so that they match the bit rates of the speech and channel encoders in the MS. Following these changes, incoming speech at the mobile is encoded using a codec mode which is suitable for the state of the up-link channel.
For down-link adaptation, the quality indicator from a down-link quality measurement unit is used to generate a Mode Request MRd in a down-link Mode Request generator in a link adaptation block located in the MS. The down-link Mode Request generator generates the Mode Request by comparing the down-link quality indicator with certain thresholds. The Mode Request indicates the preferred codec mode for the down-link path (in the down-link direction). The Mode Request is transmitted in-band across the air interface to the network where it is fed into a down-link mode control unit. This mode control unit generally grants the requested mode. The Mode Request may be overridden depending on possible constraints from network control. The Mode Request is then applied so that an incoming speech signal in the network is encoded in a way similar to that described above for the up-link channel.
Both for up-link and down-link, information about the presently applied codec mode (mode indication) is transmitted in-band. The Mode Indicator is channel coded using a block code which is separate from speech data. In the receiving end, this code is first decoded in the channel decoder to determine the right decoding mode for speech data. At the multi-rate channel decoder, the Mode Indicator is decoded and applied for channel decoding and speech decoding.
The thresholds which are used to compare against measured channel quality estimates and to decide when to change encoding modes are sent to the MS by the BTS. In total, there are 8 codec modes and a set of 4 codec modes are selected from these. Only one combination (codec set) of 4 particular codec modes can be used in a given direction at a given time. The codec modes differ by the proportions of the channel gross bit rate allocated between speech coding and channel coding (more robust transmission has lower bit-rate speech coding and more bits allocated to channel coding in order to deal with a larger amount of channel errors). Each codec set comprises a set of thresholds. The thresholds represent the optimum switching points between sequential speech codec modes. They are set to apply when the mode with the higher speech coding bit rate suffers from channel errors so much that the subjective quality drops to the level of the speech codec with the lower bit rate in the same channel conditions (fewer channel errors in the lower mode because of more channel coding). Information enabling the codec modes to be set is sent by the BTS to the MS at the beginning of communication and from time to time thereafter. For example, the set of 4 codec modes may be changed on hand-over. This may be in response to hand-over to a BTS that does not support the codec modes used previously.
The GSM AMR speech system operates on the existing GSM full-rate and half-rate speech traffic channels where the gross bit rates are fixed.
The codec mode in up-link may be different from the one used in down-link, while the channel mode, whether half-rate or full-rate, must be the same.
In the GSM AMR speech service, control is centralised in the network and the MS is not allowed to make autonomous decisions about the codec mode in the up-link direction. However, the network is able to determine the down-link codec mode. In doing this, the network may use the Mode Request or may use other information. The MS must be capable of operating in all possible codec modes, that is encoding and decoding for all 8 codec modes has to be implemented in the MS. The network, on the other hand, does not have to provide all possible modes because the BTS sets the mode-set (a maximum of 4 modes) for the MS and there is no mandatory requirement for it to be capable of encoding and decoding in every mode.
Other forms of codec adaptation exist. One form, referred to as source-controlled codec adaptation, is based on characteristics of the input signal. In this case, the bit rate and the parameters describing the speech signal are selected by the speech encoder. This may depend on whether the input signal is voiced/unvoiced and/or on the level of background noise. There exist well-known methods to adapt codec operation according to these parameters. A simple method is to measure the energy of the speech signal during speech pauses before it enters the speech encoder and to use the measurement obtained to select a higher or a lower bit rate. If the power is higher, higher bit rate encoding could be chosen and this would suggest a higher level of background noise.
An example of a system that uses source-controlled adaptation is the IS-95 CDMA mobile telecommunications system. In this system, control is not centralised and the mobile station and the network can each change the bit rate of the speech and channel encoders autonomously according to the speech signal which is input at the respective part of the system. The system does not have channel quality-controlled adaptation.
The reason for providing source-controlled codec adaptation is that coded speech quality degrades as a function of the bit rate, and the degradation is more distinct in the presence of the background noise. Therefore, the optimal switching point between different codec modes is different depending of the presence of background noise. However, in systems having centralised control for adaptation, where a mobile station receives and must obey instructions to change its speech encoding bit rate from a mode control unit in the network, implementation of a mobile station controlled source-controlled codec adaptation, such as that used in IS-95, is not possible.
The current implementation of the GSM AMR speech service is an adaptive system controlled by channel quality measurement. There is no source-controlled adaptation.